Join Date: Apr 2010
Location: ~*~ NEBRASKA ~*~
My horse too has a bad scar across the front of a leg, he's had it since he was a foal and it doesn't cause issues. My vet says that usually the ones across the front aren't that bad, it's the one across the back of the leg that can really be an issue. If it were causing an issue, I doubt it would be sudden, it would be something that would come on over the course of a few days.
Sudden onset where the horse doesn't want to put any weight on can be many things. A vet does need to come out. It could be as simple as a stone bruise to a tendon/ligament injury (however there would be heat and/or swelling if this were the case). If the horse was recently shod it could be a "hot nail" (a nail that goes into the sensitive tissue) or if no shoes a farrier could have trimmed to much.
It's always best to have a vet rule it out as soon as possible however, I don't think a trip to the emergency vet is necessary unless the horse is cut and bleeding, or has a severe trauma. You say there is no heat so it is most likely a bruised sole, or other foot soreness. I think Standing in a quiet stall until a vet can come out to say for sure what it is would be the best way to go.